Influence of stagnation gradient for face support calculation in Slurry Shield Tunnelling

Abstract

In slurry shield tunnelling, a reliable face support is presumed if a sufficient support pressure is transferred and the slurry excess pressure provides efficient support pressure on the soil skeleton. The required support pressure is calculated in reference to the groundwater pressure and the earth pressure. In this paper, the sliding wedge mechanism is applied. To ensure face stability, the support pressure transfer needs to be provided within the theoretical sliding wedge at the tunnel face. Therefore, the penetration depth of the bentonite suspension as a support medium has to be kept within limits. Research shows that certain penetration is required.The stagnation gradient defines the relation of the support pressure and the penetration depth of the suspension. The gradient has a described minimum, but also an undefined but existing maximum. The German standard DIN 4126 describes two cases for the minimum at each trench depth: Either the pressure losses due to the pressure acting outside the sliding wedge are less than 5 % or the stagnation gradient is at least 200 kN/m3. If the supporting pressure gradient is below 200 kN/m3, DIN 4126 defines rough reduction factors. For a more precise observation of the stagnation gradient, this paper presents a detailed supplement to the DIN 4126 approach taking the tunnel diameter and the angle of the sliding wedge into account. The aim is to determine the minimum recommended stagnation gradient for which no increase of the slurry excess pressure is necessary due to too deep penetration.

Erfahrungen beim Tunnelbau in durchlässigem glazialen Lockergestein: offener Kies

“…The mined section was driven with a Mix-Shield that operated as TBM in rock and as slurry shield in soil and the transition zone. The used slurry mixes correspond to the literature [17,18]. Initially during driving the slurry losses were large and the slurry lost its supporting capabilities, requiring frequent changes of the slurry.…”

Erkennen geotechnischer Vorgänge an der Ortsbrust als Schlüssel zum erfolgreichen Tunnelvortrieb

“…Beim Bau der parallelen Eisenbahntunnel unter dem Großen Belt wurden in der Grundmoräne, mit großem Anteil von Silt, vier Erddruckmaschinen verwendet [6]. [8,9].…”

“…The mined section was driven with a mixed shield that oper ated as TBM in rock and as a slurry shield in soil and the transition zone. The used slurry mixes correspond to the literature [8,9]. During driving the slurry losses were large and the slurry lost its supporting capabilities, requiring…”

A novel hydro‐mechanical coupled multiphysics model for slurry penetration in sand with application to face stability analysis of shield tunnel

“…On this basis, within the framework of the limit equilibrium method, the mass force deduced from the excess pore pressure was introduced into the wedge-prism model to estimate the tunnel face stability under slurry penetration. 11,12 However, the slurry penetration was considered as a one-dimensional seepage flow, only the contribution of excess pore pressures confined in the circular range of the wedge section was considered. Moreover, the rapid pressure drop within the slurry penetration zone was simplified by using a fixed pressure reduction coefficient.…”

“…In contrast, the degradation of slurry support induced by the excess pore pressure could not be considered. On this basis, within the framework of the limit equilibrium method, the mass force deduced from the excess pore pressure was introduced into the wedge‐prism model to estimate the tunnel face stability under slurry penetration 11,12 . However, the slurry penetration was considered as a one‐dimensional seepage flow, only the contribution of excess pore pressures confined in the circular range of the wedge section was considered.…”